Bibcode
DOI
Hirzberger, J.; Bonet, J. A.; Vázquez, M.; Hanslmeier, A.
Bibliographical reference
The Astrophysical Journal, Volume 515, Issue 1, pp. 441-454.
Advertised on:
4
1999
Journal
Citations
75
Refereed citations
61
Description
The properties of the evolution of solar granulation have been studied
using an 80 minute time series of high spatial resolution white-light
images obtained with the Swedish Vacuum Solar Telescope at the
Observatorio del Roque de los Muchachos, La Palma. An automatic tracking
algorithm has been developed to follow the evolution of individual
granules, and a sample of 2643 granules has been analyzed. To check the
reliability of this automatic procedure, we have manually tracked a
sample of 481 solar granules and compared the results of both
procedures. An exponential law gives a good fit to the distribution of
granular lifetimes, T. Our estimated mean lifetime is about 6 minutes,
which is at the lower limit of the ample range of values reported in the
literature. We note a linear increase in the time-averaged granular
sizes and intensities with the lifetime. T=12 minutes marks a sizeable
change in the slopes of these linear trends. Regarding the location of
granules with respect to the meso- and supergranular flow field, we find
only a small excess of long-lived granules in the upflows.
Fragmentation, merging, and emergence from (or dissolution into) the
background are the birth and death mechanisms detected, resulting in
nine granular families from the combination of these six possibilities.
A comparative study of these families leads to the following
conclusions: (1) fragmentation is the most frequent birth mechanism,
while merging is the most frequent death mechanism; (2) spontaneous
emergence from the background occurs very rarely, but dissolution into
the background is much more frequent; and (3) different granular mean
lifetimes are determined for each of these families; the granules that
are born and die by fragmentation have the longest mean lifetime (9.23
minutes). From a comparison of the evolution of granules belonging to
the most populated families, two critical values appear for the initial
area in a granular evolution: 0.8 Mm^2 (d_g=1.39") and 1.3 Mm^2
(d_g=1.77"). These values mark limits characterizing the birth mechanism
of a granule, and predict its evolution to some extent. The findings of
the present work complement the earlier results presented in this series
of papers and reinforce with new inputs, as far as the evolutionary
aspects are concerned, the conclusion stated there that granules can be
classified into two populations with different underlying physics. The
boundary between these two classes could be established at the scale of
d_g=1.4".